1. Field of the Invention
The present invention generally relates to semiconductor lasers and, more particularly, is directed to a compound semiconductor laser having AlGaAs/GaAs etc. based compound semiconductor of the superlattice structure.
2. Description of the Prior Art
In the conventional semiconductor lasers, the gain waveguide function is formed therein in order to lower the threshold current. That is, a current blocking unit for limiting a current is provided to concentrate the current on an oscillation region of an active layer of the laser, thereby forming a portion having a large carrier density, that is, a portion where the gain distribution is increased rapidly. When this type of gain-guided type semiconductor laser is formed, the current is limited by providing a current limiting unit for limiting a current path which is normally formed by selectively forming a high resistance region by the ion implantation process using materials such as proton or boron or by forming a pn junction current cut-off region.
However, according to the above-mentioned methods, the current cannot be concentrated sufficiently so that a threshold current Ith cannot be reduced sufficiently and that the characteristics of the semiconductor laser cannot be stabilized.
In order to perform this current limitation surely with a good reproducibility, the same assignee of the present application has proposed in Japanese Laid-Open Patent Publication No. 63-56981 a compound semiconductor laser constructed in a manner that the current limitation is performed by using a phenomenon that, when the III/V compound such as Al.sub.x Ga.sub.1-x As group or In.sub.y Ga .sub.1-y As group (0.ltoreq.x,y&lt;1) is formed by epitaxial growth by the organometalic chemical vapor deposition process, that is, the so-called MOCVD process, the carrier density differs depending on the crystalline plane orientations of the substrate on which the epitaxial growth is performed also when the MOCVD process is performed under the same condition, thereby concentrating the sufficient current on the current limiting portion with a good reproducibility.
Namely, this semiconductor laser is constructed as shown in FIG. 1 that slanted surfaces 12 are formed on a major surface 1A of a substrate 21, e.g., a GaAs single crystal substrate such that a {100} crystal plane and another crystal plane different from the {100} plane are facing on the major surface 1A, and then the III/V group compound semiconductor layer, e.g., a buffer layer 22, a first cladding layer 23, an active layer 24, a second cladding layer 25 and a capping layer 27 are sequentially formed on these crystal planes of the substrate by the epitaxial growth by the MOCVD process, for example. In this configuration, the densities of the epitaxial growth region naturally vary in a range of about 10.sup.16 to 10.sup.18 cm.sup.-3 and so the carrier densities thereof vary depending on the above-described varieties of the densities, whereby a current concentration region 26 for limiting a current path can be formed by using such a fact that the difference of the electron mobilities, that is, the difference of the resistivities due to the varieties of the carrier densities becomes remarkable. Thus, by using the current concentration region 26 having a resistivity smaller than those of other regions, a region where the current is concentrated in a stripe configuration, that is, a portion having a large gain and a high density of injection carrier can be formed in the active layer 24 positioned at a tip potion of the region 26, thereby making it possible to perform laser illumination.
However, when forming the current concentration region 26 on the two slanted surfaces 12, it has been difficult to surely cross the respective current concentration regions 26 formed on these two slanted surfaces 12 at the active layer 24, and so the characteristics of the laser is likely varied to degrade yield.
On the other hand, since the epitaxial growing apparatuses using the MOCVD process etc. have recently progressed remarkably, the growth of a thin film with a thickness of about a single atomic layer has become possible when growing the compound semiconductor based on AlGaAs/GaAs group etc., so that it has become possible to form a compound semiconductor such that its carrier densities and composition change sharply toward the growing direction for every thickness about a single atomic layer.
By using this technique, there has been tried to form a semiconductor laser whose active layer is constituted by the superlattice structure, for example. In this case, impurities are introduced into both side portions of the active layer to disorder the superlattice (layered structure) into an alloy (bulky structure), namely, an intermixing state, thereby forming regions having a small refractive index at the both side surfaces of the active layer to confine the light of the transverse mode therebetween.
However, when introducing the impurities by the selective diffusion process, it has been difficult to place the region to be an alloy in an intermixing state accurately and so the manufactured semiconductor laser has been poor in reliability.